This invention generally relates to hoses and more particularly to a method for making a hose structure having an embedded electrical conductor for preventing accumulated electrostatic charge.
There are various hose applications wherein an electrostatic charge poses a safety hazard in the use of the hose as for example, in the conveyance of inflammable fuels, grain materials, paint, etc. In this circumstance, attempts have been made to incorporate an electrical conductor into the body structure of the hose such that any accumulated charge may be dispersed and the hazard eliminated. These attempts generally involve spiralling an electrical conductor into the hose structure during the hose building operation and covering it with one or more layers of braided fabric and/or elastomer as exemplified in prior U.S. Pat. Nos. 3,543,803 and 3,780,208. The wire conductor is spiralled so that it flexes in the normal use of the hose and does not impair the flexibility of the hose.
A disadvantage of this type construction resides in the fact that the conductor by virtue of its cross-section, develops a ridge in the outer plies of the hose structure. The ridge is accentuated if the hose is built via non-mandrel construction techniques and cured without benefit of a lead covering. The lead press curing method is well known in the industry and generally involves internally pressurizing lengths of the hose, running it through a lead press to apply an outer sheath of lead, and curing the resulting structure in an autoclave. Upon finishing the cure, the lead sheath is stripped off and recycled. The lead press curing method, while providing a hose having an improved surface appearance, increases the manufacturing costs of a highly competitive product. Attempts at manufacturing hose having an embedded conductor using non-mandrel techniques and eliminating the lead press curing method have resulted in an increase in elastomeric covering material for the protection of the conductor wire. This of course has resulted in an increase in weight and cost of the product while not completely eliminating the unsightly ridge on the surface of the hose.
Other attempts at incorporating a static ground conductor in a hose structure, include various type flat or ribbon conductors as exemplified in U.S. Pat. No. 3,555,170 wherein copper threads and elastic yarn are woven into a tape or ribbon that is ultimately laid along the hose length, parallel to the hose axis. The ribbon conductor will thus stretch to accommodate hose elongation in the normal use of the hose. While this type conductor eliminates the ridge appearance and other disadvantages of spiralled conductors, it has the disadvantage of being difficult to handle in the course of a hose manufacturing process and it is a more expensive conductor to produce, thus again increasing the cost of a highly competitive product.
A solution to the above-mentioned problems may be had by incorporating a multiple wire braided conductor into the body structure of the hose as taught in U.S. Pat. No. 3,042,737. In this instance, the embedded conductor comprises a bundle of small diameter, braided wire conductors in a flattened configuration such as to exhibit the smallest cross-sectional area as the conductor is laid along the longitudinal axis of the hose. The present invention teaches a method for making a flexible hose structure incorporating a braided static conductor that remains invisible on the surface of the hose while maintaining the original flexibility of the hose. The invention takes advantage of non-mandrel construction techniques while also eliminating the lead press curing method. This is accomplished in apparatus comprising at least two pair of friction rollers placed immediately ahead of the braid point of a braiding machine as the hose is being built. The rollers are driven at different speeds so that the first of the pair operate to pull a braided bundle of wire conductors off of a supply spool while at the same time compressing the bundle against the second of a pair of rollers which are driven at a slower rotational speed. The wire bundle is thus shortened in the on-axis direction of the bundle and spread in the off-axis direction transversely of the bundle axis to form a flattened ribbon of wire conductors that are discharged at approximately the same speed as an uncured elastomeric hose tube moving relative to the pairs of rollers through the braider.